Recently, the reduction of the size and weight of power transformers used for a switching power supply, etc., has been required and for this reason, the driving frequency has been expanded to a high frequency range beyond the conventional range of from 100 KHz to 500 KHz, and the future application in the MHz range has been examined. However, because an eddy current loss, which becomes a particular problem in the high frequency range among the power loss, is proportional to the square of the driving frequency, the power loss increases with the increase of the frequency, so that exothermy cannot be neglected. Various proposals have been made in the past to solve such a problem. Among them, some represent that high performance can be expected for a soft ferrite sintered body necessary for accomplishing high characteristics by a fine structure in which texture fluctuation of individual crystal particles is less, and which has small amounts of impurities and has a high density.
Particularly because the reduction of the crystal particle diameter of the sintered body is effective, Japanese Unexamined Patent Publication (Kokai) No. 1-224265 teaches to produce a fine sintered body having a mean crystal diameter of 5 .mu.m so as to restrict the power loss in the high frequency range. Another improvement example which uses fine powder prepared by a hydrothermal synthesis method as the raw material powder has been reported (refer to "Nikkei Material & Technology", 125, (1993) 24). The soft ferrite raw material powder for obtaining such a sintered body must have smaller amounts of impurities, must have uniform components and small variance of particle size distribution and must be fine particles. Therefore, improvements and development have been continued.
A method of atomizing and thermally decomposing a pickling waste liquor of steel is known as one of the iron oxide methods of obtaining soft ferrite raw materials. In this case, if various metallic ions constituting the ferrite are mixed in the pickling water liquor, a ferrite powder can be obtained by atomization and thermal decomposition. According to the ordinary atomization thermal decomposition method, however, components having a high vapor pressure such as zinc evaporate, and a homogeneous composition cannot be obtained so easily. Therefore, Japanese Unexamined Patent Publication (Kokai) No. 55-144421 discloses a production method of a soft ferrite sintered body which mixes Fe and Mn ions, which have stable high temperature thermal decomposition characteristics, as the soft ferrite raw material powder in the form of an aqueous chloride solution, prepares a mixed oxide containing Fe--Mn in a predetermined composition by atomization thermal decomposition (roasting), then mixes zinc oxide powder, and thereafter conducts pulverization, granulation, molding and sintering.
On the other hand, as a method of producing a composite oxide for the Zn-containing soft ferrite raw material powder, Japanese Unexamined Patent Publication (Kokai) No. 63-156017 discloses a method which sprays a chloride solution containing the ferrite raw materials in a predetermined composition to a fluidization roasting furnace to conduct a high temperature oxidation reaction, withdraws the powder with the decomposition products, and recovers the soft ferrite raw material powder by a dust collector.
As an improvement of Japanese Unexamined Patent Publication (Kokai) No. 63-156017 described above, Japanese Unexamined Patent Publication (Kokai) No. 1-192708 discloses a method which atomizes and mixes a high temperature high velocity gas not containing a reducing material with a mixed solution of starting metal chlorides, quickly keeps a predetermined roasting temperature and decomposes the metal chloride mixture while being accompanied by the flow of the starting materials and the decomposition products in the concurrent arrangement with the heat gas flow.
As to the purification of the chloride solution, Japanese Unexamined Patent Publication (Kokai) No. 63-315522 discloses a method which brings the aqueous iron chloride solution into contact with the roasting gas generated during this process so as to conduct heat concentration and to insolubilize the Si component in the aqueous iron chloride solution, then adds iron into this solution so as to consume the free acid within a range of pH of 1 to 3, and to insolubilize the major proportion of Al, Cr, Cu and P components in the solution, and separates and removes the insoluble matters.
According to the prior art technologies described above, however, it is difficult to produce a sintered body having a crystal particle diameter of not greater than 2 .mu.m. To produce a sintered body having such an ultra-small particle diameter, it has been necessary to use ultra-fine powder produced by the hydrothermal method and conduct sintering by hot isostatic pressing (HIP) or hot press. However, when the powder produced by the hydrothermal method is used as the starting material or when HIP and the hot press are used as the sintering equipment, the production cost becomes extremely high, and a problem yet remains unsolved in the aspect of mass-producibility.
When a method which sprays the chloride solution into the fluidization roasting furnace and conducts the high temperature oxidation reaction as the production method of the composite oxide for the soft ferrite raw material powder is employed, the strict control of the reaction time cannot be conducted after spraying of the raw material solution due to the structural limitation of the fluidization roasting furnace, and the reaction time varies from several seconds to some dozens of minutes. Therefore, though the particle size of the ferrite raw material powder and microscopic homogeneity of the composition can be improved, the problems that the particle diameter distribution is great and component deviation is great, too, remain unsolved.
In the atomization roasting method according to the prior art, the essential conditions for controlling the particle diameter of the soft ferrite raw material powder as the product and the components thereof, such as the spray method of the starting materials, control of the sprayed droplets, quenching by using a cooling medium containing H.sub.2 O and/or O.sub.2 after the reaction for a predetermined time, etc., have not been established with the exception of the condition that roasting is carried out in the vapor-containing oxidizing atmosphere. If these conditions are not stipulated, thermal decomposition and high temperature oxidation by atomization roasting become incomplete, and chlorides in amounts as great as 1 to 3% remain due to the chlorination reaction of the undecomposed matters or the reaction products. Further, evaporation scattering of zinc takes place. If control of the sprayed droplets is incomplete, variance occurs in the particle diameter of the resulting oxide powder. Therefore, these methods have not yet been complete as the production method of the Fe, Zn and Mn type soft ferrite raw material powder.
The present invention aims at providing a raw material powder, a sintered body, its production method and its apparatus which solve the problems involved in the production of a soft ferrite raw material powder which is most suitable for high frequency transformers of the recent type and has excellent homogeneity, and can particularly produce a soft ferrite core having an excellent power loss.